1. Field of the Invention
The present invention relates generally to an aspherical surface ocular lens and, more particularly, to an ocular lens employed for, e.g., a microscope and a telescope such as a binocular telescope.
2. Related Background Art
For example, in a telescope such as a binocular telescope and a microscope, an ocular lens is used for further magnifying and observing a real image formed by an objective lens. Such an ocular lens is required to sufficiently correct a variety of aberrations through a wide field of view and, in addition, to secure an eye relief (on-axis spacing between the closest-to-eye-point surface of the ocular lens and an eye point) having a sufficient length to provide excellent optical performance over a wide field of view.
Generally, an optical system of the ocular lens or the like has such a characteristic that a Petzval's sum becomes larger with a shorter focal length, and, therefore, the various aberrations such as a curvature of field are further deteriorated at the shorter focal length.
Moreover, in the typical ocular lens, only an eye relief on the order of 80% of the focal length of the whole lens system can be obtained. Accordingly, the ocular lens having a short focal length is incapable of obtaining an eye relief having a sufficient length.
As described above, the conventional ocular lens is deteriorates in terms of the various aberrations and is therefore incapable of obtaining the eye relief having a sufficient length as the focal length is decreased. Under such circumstances, a better correction of the various aberrations entails an increased number of constructive lens elements, but this results in a disadvantage of increasing the costs.